Base load power plants such as nuclear power plants and combustion-thermal power plants are preferably operated under a constant load. Peak power electrical energy requirements are basically undesirable from a standpoint of highly efficient generation of electrical energy, but must be met on a daily and seasonal routine. Peak power electrical energy demand penalizes power generation facilities basically in the form of increased capital cost and generally higher fuel cost. To make additional electrical power capacity available during peak demand periods, higher than required capacity usually exists during off-peak periods which therefore requires energy storage systems.
The periods of low power demand, thus, leave the base load power plant operating under less than optimal conditions and excess power generation capacity is available during such periods. It is very desirable in this instance to store electrical energy efficiently and at low costs, and many schemes have been proposed for such energy storage facilities. For example, "pumped hydro" and "compressed air" energy storage systems have been used. These systems store energy in the form of potential energy during off-peak hours and return the energy to the power group during peak power demand periods. Such practical schemes are dependent on geography and geology and require significant space, and, therefore, the siting of such energy storage facilities is not flexible enough for wide-spread applications.
Electrochemical energy storage systems however would have distinct advantages over other systems. Electrochemical energy storage systems are highly compact because chemical energy can be stored in a small volume. For this reason, it can be modular in construction. It also has a potential for being the most energy efficient method for storage of electrical energy since the energy conversion of electrical to mechanical and back to electrical energy is avoided.
High temperature, solid oxide electrolyte fuel cells and multi-cell generators and configurations thereof designed for converting chemical energy into direct current electrical energy at temperatures typically in the range of 600.degree. C. to 1200.degree. C. are well known and taught, for example, in U.S. Pat. Nos. 4,395,468 (Isenberg) and 4,490,444 (Isenberg). These high temperature, solid oxide electrolyte fuel cells are known to operate in two modes, namely in an electrochemical power generation mode using gaseous fuel such as hydrogen or carbon monoxide derived from reformed hydrocarbons, coal, or the like, which is converted to direct current electrical energy, and in an electrochemical power usage mode using steam and carbon dioxide which is converted via electrolysis into oxidizable fuels such as hydrogen or carbon monoxide, respectively, as generally taught in U.S. Pat. No. Re. 28,792 (Ruka, et al.). This capability of the high temperature, solid oxide electrolyte fuel cells is unique among fuel cell types and is due to the fact that the solid oxide electrolyte fuel cells are solid state devices which operate in a temperature range of about 600.degree. C. to 1200.degree. C. At these temperature levels, the thermal energy is high enough that electrolysis can proceed without using noble metal catalysts at the electrodes, and the laws of thermodynamics predict, and are confirmed by experiment, a reduced electrical power requirement for electrolysis as compared to low temperature electrochemical batteries. For example, sodium-sulfur batteries that operate at about 350.degree. C. have been investigated for use in storing off-peak electrical energy.
Producing fuels, such as hydrogen fuel gas, by electrolysis is a basic requirement for an electrochemical energy conversion and storage system. The energy storage in the form of hydrogen fuel gas through the electrolysis of steam, however, would require bulky gas storage facilities and would restrict the siting of this type of energy conversion and storage system. Therefore, there is a need to convert the bulky hydrogen energy carrier into another form of energy carrier which is not bulky and can be stored in a compact arrangement. At the appropriate demand time, this potential energy can be called into service by reconversion to hydrogen as a fuel to a fuel cell generator to generate electrical energy.
What is needed is an efficient and compact method of storing electrical energy in the form of chemical energy that is relatively compact and can be stored indefinitely for later reconversion to electrical energy for electrical power transmission.
It would be advantageous and it is an object of the invention to derive an energy storage and conversion system from high temperature, solid oxide electrolyte fuel cells operating in two modes, i.e., 1. an electrolysis mode for electrical energy storage and 2. a fuel cell mode for electrical energy recovery, using iron metal and iron oxide (Fe/FeO) beds as the energy storage mediums. In the first mode of operation or energy storage mode of the invention, a high temperature, solid oxide electrolyte fuel cell powered with electrical energy supplied from an electrical power plant electrolyzes incoming H.sub.2 O (steam) to H.sub.2 and O.sub.2. The H.sub.2 gas thus produced is fed to a heated Fe/FeO storage reactor bed containing iron oxide (FeO) which is reduced to iron metal (Fe) as the energy storage medium, and H.sub.2 O (steam) is produced. The H.sub.2 O (steam) produced in the energy storage reaction is recirculated to the solid oxide electrolyte fuel cell for electrolysis and the H.sub.2 produced is recirculated to the Fe/FeO storage reactor bed again and again until there is substantially complete conversion of FeO to Fe.
In the second mode of operation or the energy recovery mode of the invention, the Fe/FeO storage reactor bed now containing iron metal (Fe) reduces incoming H.sub.2 O (steam) to H.sub.2 gas which is an oxidizable electrochemical fuel for electrical power generation in a high temperature, solid oxide electrolyte fuel cell. The H.sub.2 gas produced is fed to a high temperature, solid oxide electrolyte fuel cell and is electrochemically reconverted into electrical energy and H.sub.2 O (steam). The H.sub.2 O (steam) produced in the electrochemical oxidation is recirculated to the Fe/FeO storage reactor bed and converted to H.sub.2 again and again until there is substantially complete conversion to gaseous fuel and discharging of Fe to FeO. The second mode of operation can also use over the fence fuel such as natural gas by using the Fe/FeO beds as reformers, thereby increasing the efficiency of the energy storage and conversion system of the invention, as more completely described in U.S. patent application Ser. No. 08/378,298 (Isenberg) entitled A Hydrocarbon Reformer For Electrochemical Cells filed currently herewith which is incorporated by reference herein, in its entirety.
The iron/iron oxide (Fe/FeO) beds therefore undergo multiple cycles of oxidation and reduction reactions according to Equation (1). ##STR1##
Thus, a first storage mode involves conversion of direct current electrical energy, typically supplied from an electrical power plant during off-peak hours, by the electrolysis of steam coupled with chemical energy storage by the reduction of iron oxide by hydrogen gas in an iron oxide/iron metal storage bed and a second recovery mode involves the recovery of direct current electrical energy from the iron oxide/iron metal storage bed by the oxidation of iron by steam to iron oxide and hydrogen gas which is subsequently electrochemically oxidized into direct current electrical energy. The two modes of operation of the system are performed by high temperature, solid oxide electrolyte fuel cells (SOFC). The system can also be operated in a third energy generation mode by using the iron oxide/iron metal beds as a reformer for natural gas also in conjunction with high temperature solid oxide electrolyte fuel cells.